Heterogeneous catalysis processes, using metal catalysts, are of commercial importance in a large number of chemical and petrochemical processes. In all cases, the economic performance of the processes depends, to a greater or lesser extent, on the activity of the catalyst, its selectivity towards the desired reaction product, and the cost and complexity of preparation of the catalyst in its most advantageous form for use in the particular process under consideration. For most efficient catalysis, the catalyst should have large metallic surface area, i.e. a large surface to bulk ratio.
This is achieved by producing the catalyst comprising individual occurrences of metal atoms (monoatomic) where most of the metal atoms are atoms in the zero-valent form and wherein the individual occurrences of metal atoms (monatomic) form aggregates or clusters, exhibiting molecular metal properties as opposed to colloidal, bulk metal properties. As used herein, the term "cluster" refers to metal atoms weakly or strongly coupled, through space or through a support a significant proportion of the metal atoms being in the zero-valent state and generally separated by a distance of six Angstrom (.ANG.) or less. Such a cluster includes any aggregation of two or more metal atoms, of the same or different species, regardless of whether they occur in substantially one dimensional form (i.e. a chain of metal atoms), or two dimensional form (i.e. a planar arrangement), a spiral arrangement or a three dimensional structure.
When bulk metals, especially transition metals, are vaporized e.g. by resistive heating, the initially formed vapor is in the monatomic condition. Very rapidly indeed, under normal conditions, the single metal atoms agglomerate into small clusters on a surface, and then very rapidly bulk, colloidal metal is formed by further agglomeration.
U.S. Pat. No. 4,292,253, Ozin and Francis, issued Sept. 29, 1982, describes a process for preparation of a catalyst in which the catalytic metal is present, in significant amounts, in small cluster form and is stable at or near room temperature. The process described involves the generation of vapors of the metal in a high vacuum environment and in the vicinity of a liquid polymer having reactive groups, so that the metals are effectively "trapped" by the polymer in monatomic or small cluster form and prevented from recombining to form colloidal metal. This work showed that the metal was anchored at specific reactive sites of the polymer structure, with further depositions of metal atoms causing growth in the cluster sizes rather than creation of additional clusters at new nucleation sites.
It is known to provide solid catalysts comprising various metals deposited on an inert carbon support. The carbon support, normally in particulate form, is treated by washing with a solution of a reducible salt of the appropriate metal. Then the support carrying the salt of the metal is subjected to reducing conditions, e.g. using hydrogen, to form metal on the surface of the carbon particles. Such catalysts are useful in a variety of reactions including hydrocarbon cracking, hydrocracking, hydrocarbon reforming and the like as described by B. C. Gates, J. R. Katzer, and G. C. A. Schuit, "Chemistry of Catalytic Processes", McGraw-Hill, N.Y., N.Y. 1979 and in electro-catalytic processes, e.g. as fuel cell electrodes. As in all heterogeneous catalysts, the most active forms are those which present the largest ratio of surface atoms to internal atoms of catalytic metal, so that the presence of the metal in individual occurrences and in small cluster form on the surface of the carbon particles is desirable.